Semiconductor devices which are capable of generating and emitting light are generally categorized as light-emitting diodes and lasers. The light-emitting diodes generally emit non-coherent light whereas lasers emit substantially coherent beams of light. In general, each of these semiconductor light-emitting devices comprises a body of a semiconductor material having layers of opposite conductivity type forming a PN junction which extends to an edge of the body. Light emitting diodes have been developed which emit light either from a surface of the body (surface emitting diodes) or from an edge of the body (edge emitting diodes). However, initially, semiconductor lasers were only edge-emitting lasers. This resulted from the fact that to achieve a substantially coherent beam of light, the device requires a gain region, the forward biased PN junction, and a waveguide having spaced reflecting means for achieving the necessary feedback of the generated light to achieve coherence. The waveguide was generally formed along the PN junction between the edges of the body with the reflecting means, such as mirrors, being formed at opposite edges of the body. A disadvantage of the edge-emitting laser is that the emitting area is small resulting in a beam having a relatively large divergence angle. In order to reduce the divergence angle of the emitted beam, and thus achieve a more collimated beam of light, it is desirable to have a large opening through which the beam is emitted from the device. A surface-emitting device would provide such a larger opening.
Several attempts have been made to form surface emitting lasers. One attempt is a microlaser which is described in the articles "Low Threshold Electrically Pumped Vertical-Cavity Surface-Emitting Microlasers" by J. L. Jewell et al, Electronic Letters, Vol. 25, 1989, pg. 1123, "Top Surface Emitting GaAs Four-Quantum-Well Lasers Emitting at 0.85 um", by Y. H. Lee et al, Electronic Letters, Vol 26, 1990, pg. 710, and "Fabrication of Microlasers and Microresonator Optical Switches" by A. Scherer et al., Applied Physics Letters, Vol. 55, 1989, pg. 2724. In the microlaser, a few quantum wells are placed at the center of a one wavelength long cavity sandwiched between two stacks of quarter-wave interference mirrors. One of the stacks is doped n-type and the other is doped p-type to form a p-i-n junction. When the p-i-n junction is forward biased, double injection into the active region, which may contain quantum wells, results in radiative recombination. However, because the active region is so short, high quality mirrors must be fabricated to increase the number of passes the radiation must make through the inverted population to achieve threshold for lasing. The high reflectance of the mirrors reduces the power transmitted out of the device. Also, possible absorption and scattering losses reduce the external quantum efficiency.
Another type of surface emitting laser is the distributed-feedback graded-index, separate-confinement heterostructre (DFB GRINSCH). This type of laser is described in the articles "Efficient, High-Power (&gt;150 mW) Grating Surface Emitting Lasers" by G. A. Evans et al, Applied Physics Letters, Vol. 52, 1988, pg. 1037 and "Coherent, Monolithic Two-Dimensional (10.times.10) Laser Arrays Using Grating Surface Emission" by G. A. Evans et al, Applied Physics Letters, Vol. 53, 1988, pg. 2123. This device includes a grating outside the active region within the plane of the pn junction. The grating couples the light transversely to the surface by second order diffraction. The extent of the grating outside the active region overcomes the aperture constraints of the edge-emitting laser in its worst direction, i.e. where the thinness of the junction causes the largest beam divergence. In the other direction, i.e. parallel to the grating ruling, the beam divergence is not changed and therefore is several degrees wide. However, this type of surface emitting laser is complex in structure and relatively large in size since the grating is outside of and to one end of the active region of the device in which the light is generated.